Power devices are widely used to carry large currents at high voltages. One important power device is the power rectifier. As is well known to those having skill in the art, a rectifier allows current flow freely in one direction and blocks current flow in the other direction.
Modern power systems tend to operate at increasingly higher frequencies. High frequency operation allows smaller size components to be used in a power system and allows more efficient system design. Accordingly, modern high frequency power circuits require power rectifiers with fast switching characteristics.
The P-I-N rectifier was one of the first semiconductor devices developed for power applications. In a P-I-N rectifier, an intrinsic ("I") semiconductor layer is formed between P- and N-type semiconductor layers. The intrinsic region need not be truly intrinsic as long as its resistivity is relatively high compared to the N-type region. The P-I-N rectifier is typically formed by growing a lightly doped N-type region on a heavily doped N+ region, resulting in an N+N-P+ structure. The intrinsic region is also referred to as the "drift" region.
In operation, the intrinsic region is flooded with minority carriers during forward conduction. The resistance of the intrinsic region becomes very small during current flow, allowing the rectifier to carry high current densities during forward conduction.
The P-I-N rectifier has been the dominant rectifier for high-voltage power systems, where the device must withstand over 100 volts in the reverse direction. Unfortunately, two important drawbacks of the P-I-N rectifier limit its applicability. First, when a P-I-N rectifier is turned on at high speed, its forward voltage drop has been found to initially exceed its voltage drop during steady state current conduction. This phenomena is called "forward voltage overshoot during the turn on transient", and results from the existence of the highly resistive intrinsic region. A high forward overshoot voltage in the P-I-N rectifier can be a serious problem in power circuits because this voltage may appear across the emitter-base junction of a bipolar transistor used in the circuit, and may exceed the transistor's breakdown voltage.
A second and more serious drawback of the P-I-N rectifier is its poor reverse recovery characteristics. Reverse recovery is the process by which the rectifier is switched from its "on" or forward conduction state to its "off" or reverse blocking state. To undergo this transition, the minority carrier charge stored in the intrinsic region during forward conduction must be removed. Removal of the stored charge during switching produces a large peak reverse recovery current, and results in long reverse recovery time. The large peak reverse current of the P-I-N rectifier can cause power loss and stress in the circuit, and the long reverse recovery time limits the operating frequency of the power system. The switching characteristics of the P-I-N rectifier have been improved by lifetime control techniques, such as by the introduction of recombination centers in the intrinsic region. However, the reduction in lifetime results in a higher forward voltage drop and a larger reverse leakage current. A detailed discussion of the theory and operation of P-I-N rectifiers may be found in section 8.1 of the textbook entitled Modern Power Devices by coinventor B. Jayant Baliga, published by John Wiley & Sons, Inc., 1987, the disclosure of which is hereby incorporated herein by reference.
Schottky barrier rectifiers have also been used in power circuits. As is known to those having skill in the art, a Schottky rectifier produces rectification as a result of nonlinear current transport across a metal-semiconductor contact. The potential barrier responsible for this behavior was ascribed to the presence of a stable space-charge layer by Schottky in 1938. In a Schottky power rectifier, the dominant current flow is by thermionic emission. Reverse blocking takes place by introducing a depletion layer into the semiconductor. The optimization of the characteristics of the Schottky power rectifier requires a tradeoff between forward voltage drop and reverse leakage current. As the Schottky barrier height is reduced, the forward voltage drop decreases, but the leakage current increases and the maximum operating temperature decreases. The ultimate limiting factor which determines the choice of the Schottky barrier height is typically the power dissipation required in the rectifier.
Since forward current transport in the Schottky rectifier occurs primarily through majority carriers, these devices exhibit an extremely fast reverse recovery behavior. Furthermore, there is typically no forward overvoltage transient as experienced with P-I-N rectifiers. A detailed discussion of the theory and operation of Schottky barrier rectifiers may be found in Section 8.2 of the above cited textbook by coinventor Baliga, the disclosure of which is hereby incorporated herein by reference.
In order to obtain the desirable characteristics of the P-I-N rectifier and the Schottky rectifier while limiting the drawbacks of both, it has been proposed to merge a Schottky rectifier with a P-I-N rectifier. The Merged P-I-N/Schottky rectifier (also referred to as an "MPS rectifier"), merges a Schottky contact region with a planar P+N junction region. The MPS rectifier is described in an article by coinventor Baliga entitled Analysis of a High Voltage Merged P-I-N/Schottky (MPS) Rectifier, IEEE Electron Device Letters, Vol. EDL-8, pp. 407-409, 1987; in an article by coinventor Baliga and Chang entitled The Merged P-I-N/Schottky (MPS) Rectifier: A High-Voltage, High-Speed Diode, Proceedings of the IEDM, pp. 651-661, 1987; and in an article by the present inventors entitled Optimization of the MPS Rectifier Via Variation of Schottky Region Area, Proceedings of the Third International Symposium on Power Semiconductor Devices and IC's, pp. 109-112, April 1991. The disclosure of all three articles is hereby incorporated herein by reference. FIG. 1 herein is a reproduction of FIG. 1 of the last mentioned article, and illustrates the unit cell structure for a merged P-I-N Schottky rectifier.
In the merged P-I-N/Schottky rectifier, the P-I-N region injects minority carriers, to thereby reduce the series resistance in the drift region. As a consequence, the forward voltage drop for the MPS rectifier remains low, even at high current density. The Schottky region also reduces the stored charge in the drift region and results in high switching speeds.
Unfortunately, the MPS rectifier exhibits a high voltage drop at large current densities, due to the smaller area P+N junction compared to a pure P-I-N rectifier. Also, the leakage current in the MPS rectifier, when fabricated using planar diffusion to form the P+N junction, increases with increasing reverse bias voltage. This increase in leakage current may become substantial when the Schottky region increases and the planar P+N junction no longer provides enough pinch-off effect to suppress the leakage current through the Schottky interface. The on-state and reverse blocking characteristics of the MPS rectifier can be improved by increasing the P+N junction area in the device. Unfortunately, an increase in P+N junction area results in degradation of the switching performance of the MPS rectifier, due to the resultant reduction in Schottky area.
One attempt to improve the operation of the MPS rectifier is described in U.S. Pat. No. 4,982,260 to Chang, coinventor Baliga and Tung, entitled Power Rectifier with Trenches, the disclosure of which is hereby incorporated herein by reference. The required forward bias voltage is reduced by providing a plurality of trenches between the P-I-N regions of the device. A Schottky contact is formed at the bottom of each trench. Alternatively, as shown in FIG. 14B, reproduced herein as FIG. 2, P+ regions may be formed at the bottom of each trench and a Schottky contact may be formed on the substrate face between the trenches. Unfortunately, these structures may suffer from the same poor leakage current and on-state characteristics associated with the conventional MPS rectifier without trenches.